The type of load cell weighing scale with which this invention is concerned typically comprises a load-receiving structure which is operatively connected through a lever system or other force-transmitting system to a load cell to measure the weight of a load on the load-receiving structure. In this type of scale, the load cell is subject to damage by dynamic and/or static overloads.
To avoid such load cell damage, various devices have previously been proposed to protect or isolate the load cell from the overload. Examples of such prior devices are shown in FIG. 1 of the drawings herein and in U.S. Pat. No. 3,561,553 issued on Feb. 9, 1971 to O. J. Blubaugh, U.S. Pat. No. 3,269,472 issued on Aug. 30, 1966 to R. E. Bell, and U.S. Pat. No. 3,502,164 issued on Mar. 24, 1970 to T. Akuta et al.
Some prior load cell overload protection devices, such as the one shown in FIG. 1 of the drawings herein, employ a preloaded spring. Although this type of overload protection device is of generally simplified construction, it is not without its shortcomings.
In the prior load cell overload protection device shown in FIG. 1, the preloaded spring is indicated at A. The load cell protected by spring A is indicated at B and is suspended between a fulcrumed beam C and spring A. Load cell B is connected through a loop and pin type tension joint D, a further spring E and an isolation mass F (i.e. a heavy metal block) to one end of beam C. The other end of load cell B is connected through another loop and pin type tension joint G and a link P to one end of spring A.
The other end of spring A is tied to ground by being seated against a rigid, fixed structural member H which is bolted to or seated on the scale-supporting floor. A load-receiving structure, such as a hopper J, is suspended from beam C which is swingable about a fulcrum K between upper and lower stops L and M. With this construction only tensile forces are applied to load cell B.
Spring A, after assembly with stop plate H and the other scale components is pre-loaded against ground to seat a stop member N, which is secured to link P, against member H. For normal loading the force exerted by spring A is greater than the pull applied to load cell by a load-induced force. Stop member N therefore remains in engagement with member H to provide a stiff joint or connection tying load cell B to ground so that tension developed by a load in hopper J is transmitted through load cell B and directly to ground through the engaging members N and H. Spring E absorbs high frequency shock or drop energy during normal loading.
When an excessive load is applied to hopper J, spring A deflects causing stop N to come off its seat on member H and allowing beam C to swing up to strike stop L. The force exerted by the abnormal load is therefore diverted through stop L and is not applied to load cell B. The spring rate of spring A and the adjustable gap at stop L are such that the load cell capacity is not exceeded as spring A deflects to allow the beam to strike stop L under the influence of excessive loading.
One major disadvantage of the foregoing load cell overload protection arrangement is that spring A must first be assembled with the other scale parts and tied to ground by way of member H before it can be adjusted to the desired preload force. This structural characteristic requires preloading of spring A to be set in the field (i.e. at the site of installation) after the scale is assembled and the preload spring is tied to ground.
Another major shortcoming of the load cell overload protection system shown in FIG. 1 is that the preload force exerted by spring A will change with settlement or yielding of the scale housing, support floor or any other parts on which the structural member H is supported.